It is known to produce elastomeric polyurethane plastics by reacting linear polyhydroxyl polyethers or polyesters having molecular weights of less than 8000 with an organic diisocyanate and thereafter curing the reaction product with a cross-linking or chain-extending agent. The second step of the procedure may be accomplished by either adding the cross-linking or chain-extending agent to the reaction product and pouring or casting the resultant mixture into a mold where curing is effected with the addition of heat, or by conducting the reaction in a kneader until a friable material is obtained and then curing the material by pressing it into heated molds.
One problem associated with the use of linear polyhydroxyl polyethers is that the resultant final products have relatively poor green strength (a measure of the strength of the product upon removal from the mold). Green strength is generally evaluated by removing a test panel from a mold after a specified residence time, typically about 30 minutes, and immediately folding the panel tightly over upon itself. The folded edge is then examined for crazing or cracking and rated. Polyether-based cast elastomers often resemble sliced cheese upon removal from the mold and when folded, act in a manner one would expect of folded cheese.
It is known in the art to add various fillers to polyurethanes and polyurethane prepolymers in order to increase thixotropy (see, e.g., U.S. Pat. No. 3,607,822); to stabilize against hydrolysis (see, e.g., U.S. Pat. No. 3,716,502); and to improve tear strength (see, e.g., U.S. Pat. No. 4,062,825). Specifically, silica and other inorganic additives are known to be used as fillers in polyurethanes. Generally, however, the amounts of silica and other inorganic additives used in the art are relatively high, e.g., as high as 50 parts per 100 parts of prepolymer.
It is an object of the present invention to overcome the low green strength generally encountered with polyether-based cast elastomers.